1. Field of the Invention
The present invention relates to a magnetic resonance imaging (MRI) apparatus and a radio frequency (RF) coil assembly utilized therein, and more particularly, to an RF coil assembly for an MRI apparatus that can quickly and easily position an upper RF coil unit at a close and prescribed distance apart from an object.
2. Background of the Invention
An MRI apparatus is an image diagnosis apparatus that can magnetically excite nuclear spins of an object placed in a static magnetic field by applying radio frequency signals (RF pulses) having the Larmor frequency and that can reconstruct image data based on the magnetic resonance (MR) signals induced in accordance with the excitation. Since an MRI apparatus can acquire various diagnosis data, such as, diagnostic anatomy data, diagnostic biochemistry data and diagnostic function data, it is now inevitably used in various image diagnosis fields.
It is required for an MRI apparatus to detect MR signals acquired from a living body in order to generate image data. MR signals are detected through a radio frequency coil unit (hereinafter simply referred to as an “RF coil unit”) provided in the MRI apparatus. Generally, the RF coil unit includes a transmitting RF coil for exciting nuclear spins of diagnostic portion of an object by irradiations of RF pulses, and a receiving RF coil for detecting the MR signals from the excited nuclear spins. Of course, it is possible to irradiate (i.e., transmit) RF pulses and detect (i.e., receive) MR signals through the same RF coil unit, since an excitation timing of RF pulses differs from a detection timing of MR signals timing.
To reconstruct image data of good quality in an MRI apparatus, it necessary to detect weak MR signals acquired from a living body at high sensitivity. Accordingly, it is desired to place an RF coil unit as close as possible to a diagnostic portion of an object in order to effectively detect MR signals.
Conventionally, in an MRI apparatus for performing radiography of a whole body of an object, a whole body surface of the object is covered with a flexible RF coil unit so as to directly attach to the body surface in order to detect weak MR signals with high sensitivity. However, the conventional method has a problem that an object receives an oppressive sense of isolation, since the whole body of the object is covered by the RF coil unit. In particular, an object may be psychologically stressed when a phased array coil is utilized, since the phased array coil is formed of a plurality of RF coil units arranged in a body axis direction of the object and a total weight of the plurality of RF coil units is large.
To reduce or avoid such discomfort for an object, a method has been proposed, for example in a Japanese Patent Application Publication 7-100123, for maintaining flexible RF coils at a predetermined distance from a facing body surface of an object using a coil unit supporting unit.
FIG. 10 illustrate an internal background art for the applicant of the present invention in which an upper RF coil unit is placed on an object by using a supporting member for the upper RF coil assembly. Firstly, an operator adjusts a position of an object 150 so that a diagnostic portion of the object is located on a lower RF coil unit 36 fixed to a plate 35. After adjusting a position on the lower RF coil unit, a coil unit supporting member 37 of a cylindrical configuration is placed so as to cover the object 150. Further, a flexible upper RF coil unit 38 is placed so as to cover an upper surface of the cylindrical coil unit supporting unit 37. Then, the operator performs adjusting operations so that a whole surface of the upper RF coil unit 38 is faced opposite a whole surface of the lower RF coil unit 36. When both the coil unit supporting unit 37 and the upper RF coil unit 38 are placed so as to surround the object 150, a top plate 35 is moved along a body axis (i.e., z-axis) direction of the object 150 and is positioned in a radiography field (i.e., field of view (FOV)) formed in a gantry (not shown) for the MRI apparatus. By using the upper RF coil unit 38 placed over the object 150 and the lower RF coil unit 36 placed under the object 150, MRI radiography (imaging) is performed.
In the example shown in FIG. 10, the coil unit supporting unit 37 and the upper RF coil assembly 38 are independently constructed. Thus, two step operations are needed to place the upper RF coil unit 38 over the object 150 so as to cover a diagnostic portion of the object 150. A first step is to attach a coil unit supporting unit 37 over an upper surface of the diagnostic portion of the object 150, and a second step is to provide the upper RF coil unit 38 onto an upper surface of the coil unit supporting unit 37. Consequently, this method takes much more time and labor for placing the upper RF coil unit 38. To overcome this problem, it will be considered to construct an upper RF coil unit 38 and a coil unit supporting unit 37 as a unified body. However, such a unified body undesirably needs a larger storing space to pack it.